Ions are formed when atoms gain or lose electron/s. The atoms that do not participate in this process, thus have either more or less electrons than ions. Let us understand this with an example. Sodium is a metal and has an atomic number of 11. Thus, sodium atoms have 11 electrons. It has an electronic configuration of `1s^2, 2s^2, 2p^6, 3s^1` . To form an ion, sodium loses the lone 3s electron and becomes a cation, `Na^+` . The cation has only 10 electrons, as compared to a sodium atom that has 11 electrons. Similarly, chlorine has 17 electrons in non-ionized state. A chloride ion, `Cl^-` , is formed when chlorine atom gains 1 additional electron. Thus, a chloride ion has 18 electrons, while a chlorine atom has 17 electrons.
Thus, the electron gain or loss causes the ions to have more or less electrons as compared to regular atoms of the same element.
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